Electroluminescence is the emission of light from a crystalline phosphor due to the application of an electric field. A commonly used phosphor material is zinc sulfide activated by the introduction of less than one mole percent of various elements such as manganese into its lattice structure. When such a material is subjected to the influence of an electric field of a sufficient magnitude, it emits light of a color which is characteristic of the composition of the phosphor. Zinc sulfide activated with manganese (referred to as a zinc sulfide:manganese or ZnS:Mn phosphor) produces a pleasant yellowish orange centered at 585 nanometers (nm) wavelength.
ZnS:Mn phosphors are characterized by high luminance, luminous efficiency and discrimination ratio, and long useful life. Luminance is brightness or luminous intensity when activated by an electric field, and is commonly measured in lamberts, i.e. candelas per pi square centimeters, or in foot-lamberts, i.e. candelas per pi square feet. Luminous efficiency is light produced compared to power consumed by the device, commonly measured in lumens per watt. Discrimination ratio is the ratio of luminance in response to an "on" voltage to luminance in response to an "off" voltage.
A wide range of colors can be obtained by substituting or supplementing the manganese with other materials such as copper or alkaline earth activators, or by substituting or supplementing the zinc sulfide with other similar phosphorescent materials such as zinc selenide.
Phosphor materials can be formulated into a wide variety of electroluminescent configurations to serve numerous functions. In many electroluminescent devices the electroluminescent display is a panel which is divided into a matrix of individually activated pixels (picture elements).
Two major subdivisions of electroluminescent devices are defined in terms of the intended alternating current (AC) or direct current (DC) operating modes. In DC configurations, electrons from an external circuit pass through the pixels in the panel. In AC configurations, the pixels are capacitively coupled to an external circuit.
Electroluminescent devices are also made using either powder or thin-film phosphor configurations. Powder phosphors are formed by precipitating powder phosphor crystals of the proper grain size, suspending the powder in a lacquer-like vehicle, and then applying the suspension to a substrate, for example by spraying, screening or doctor-blading techniques. Thin-film phosphors are grown from condensation of evaporants from vacuum vapor depositions, sputtering, or chemical vapor depositions.
Two configurations to which the present invention has high applicability are the powder phosphor electroluminescent matrix and segmented display panels, intended for operation in the direct current (DC) mode. Matrix display panels can be used for a variety of applications, and in general, can find utility as substitutes for cathode ray tubes (CRTs), wherever CRTs are used. For example, matrix display panels can be used for such applications as oscilloscopes, television sets and monitors for computers. A particularly advantageous application for the matrix display panel is as the monitor for a microcomputer, or personal computer. By avoiding the need for a CRT, an electroluminescent matrix display panel can make a personal computer more compact and thus more easily portable.
Segmented display panels find utility for example as alphanumeric displays in such apparatus as digital clocks; pocket calculators; and gasoline pump indicators.
In manufacturing DC electroluminescent displays, it is necessary to electrically stimulate the phosphor of the display in a process that is known as "forming." The electrical process of forming is required to provide a continuous film in the phosphor that will luminesce with maximum intensity at a particular desired operational voltage. This forming process has been used with powder phosphor electroluminescent panels manufactured in accordance with the processes described in the following commonly owned patent applications:
______________________________________ Ser. No. Inventor(s) Title Filing Date ______________________________________ 752,317 Glaser Phosphorescent 7/3/85 Material For Electroluminescent Display 849,768 Glaser Phosphorescent 4/9/86 Material For Electroluminescent Display, Having Decreased Tendency For Further Forming ______________________________________
In manufacturing, it has been found necessary to form electroluminescent display panels in a twostage process. In the first stage, the panel is formed from its virgin state to provide luminescence at a voltage of about 25 volts. This first stage is known as initial forming of the panel. In the second stage, the voltage applied to the panel is increased until luminescence is provided at a desired activating voltage of, for example, 70 volts. This second stage of the process is known as final forming.
In the forming process, a voltage is placed across anode and cathode conducting electrodes disposed in stacked relation on an underlying glass substrate. When the voltage is applied to these electrodes, a current flows through the electroluminescent powder phosphor that is disposed between the electrodes. The level of voltage and current determines the speed with which the phosphor of the panel is formed from its virgin powder phosphor state to the desired state wherein a luminous film is provided to radiate light at a defined final voltage.
It is known that a substantial current is required during the initial forming stage to achieve luminescence and forming of the panel. However, the current that flows through the phosphor has the undesirable effect of excessively heating the phosphor during the forming process. Excessive heat will cause the phosphor to degrade, and will therefore result in reduced illumination and light for the panel that is finally formed. Accordingly, it has been found necessary to limit the amount of current that flows in the panel during the initial forming process to about 150 milliamps/cm.sup.2 at a voltage that is gradually increased from about 12 volts to 25 volts. During the initial forming process, the voltage and current must be very carefully controlled to limit the power applied to the panel and the resultant heating of the phosphor.
Also, if it is desired to initially form a rather large electroluminescent matrix display having, for example, 640 columns and 200 rows, it has not heretofore been possible to form all of the pixels or phosphor elements of the panel at one time. Simultaneous forming of all pixels of such a panel results in excessive heating and degradation of the phosphor. Accordingly, it has been found necessary to cycle the energization of spaced pixels or lines of pixels of the panel during the initial forming process. Thus, for example, it has been found that a matrix display panel may be initially formed by energizing for a particular time an initial set of column or row electrodes spaced about 16 electrodes apart. Thereafter, another set of electrodes is energized to allow the previous set to cool. Spaced sets of electrodes of the panel are cycled in this fashion for about 90 minutes until the panel has been initially formed to about 25 volts. Thereafter, in the final forming process, phosphor resistance is increased and voltage in excess of 25 volts is applied to the entire panel and increased to the final formed voltage. Thus, in the final forming process, the entire panel is energized and is brought relatively quickly to the desired final energization voltage for the panel.
A special electrical fixture and energization control circuitry are required to initially cycle forming voltage to the panel in a manner that provides about 150 milliamps/cm.sup.2 of current for the phosphor. Even with careful control of the applied power, some degradation of the phosphor is likely and the panel is therefore not formed in an optimum manner. Also, the sensitive control of the power during the initial forming process results in panels that have nonuniform life and luminescence characteristics.
It has been suggested by others that the initial forming process can be facilitated by disposing a layer of nitrocellulose between the conducting anodes and phosphor of the display. It has been found that this insulating interlayer of nitrocellulose decreases the amount of current required to initially form the panel by about fifty percent. However, the forming current is still sufficiently high so that rows and columns of a matrix panel must still be energized cyclically to form the panel. Accordingly, although excessive heating and degradation of the panel may be reduced, the initial forming process still requires considerable time.
Moreover, it has been found that nitrocellulose will tend to degrade and form water when it is heated in the forming process. It has been found that water within the panel contributes to degradation and undesirable further forming of the phosphor beyond the final formed voltage. This degradation and further forming of the panel results in a substantially decreased life for the panel.
Also, the organic nitrocellulose interlayer is applied to the panel by a relatively imprecise dipping process that produces an interlayer of nonuniform thickness. Also, the interlayer has a tendency to form pinholes. The pinholes result in microchannels of relatively intense current during forming and thereby contribute to undesirable heating of the panel. Finally, the dipping process must be carried out in a relatively dust-free environment. Accordingly, dipping requires a rather expensive clean room facility.
The disadvantages of the use of a nitrocellulose interlayer are so substantial that this interlayer is generally not favored in a high volume manufacturing process. Accordingly, even though it provides a desirable reduction in the amount of current for initial forming, its disadvantages discourage its use in manufacture.
Also, it has been found that a conducting sulfur nitride polymer (SN.sub.x) can form in the phosphor of a display and adversely affect the operation of the phosphor. It would be desirable to avoid the formation of this polymer and also convert any SN.sub.x polymer that is formed to a harmless substance within the phosphor.
It is therefore an object of the invention to provide an electroluminescent display panel that can be initially formed in a relatively short time and with little or no degradation of the phosphor.
It is another object of the invention to provide such a panel that is initially formed as a whole.
A further object of the invention is to provide an electroluminescent panel with a transparent inorganic insulative interlayer that is precisely formed as a thin film between the conducting anodes and phosphor of an electroluminescent panel.
Another object of the invention is to provide a panel with a metal oxide interlayer that will facilitate initial forming.
A further object of the invention is to provide an electroluminescent panel with an interlayer that is made of either aluminum oxide, magnesium fluoride, magnesium oxide, yttrium oxide, or zinc sulfide.
Another object of the invention is to provide an improved process for avoiding the formation of a sulfur nitride polymer in the phosphor and converting any of this polymer that is formed to harmless S.sub.2 N.sub.2.